The **key difference** between sensitivity and specificity is that **sensitivity measures the probability of actual positives, while specificity measures the probability of actual negatives.**

Sensitivity and specificity are two terms we come across in statistical testing. Depending on the nature of the study, the importance of the two may vary. The ideal test should be able to deliver results with 100% sensitivity and 100% specificity. However, in a practical application, it is difficult to achieve this. In most incidents, a trade between the two is essential to build a reasonable foundation to the reliability of the test results. Hence, this current article mainly focuses on the difference between sensitivity and specificity.

### CONTENTS

1. Overview and Key Difference

2. What is Sensitivity

3. What is Specificity

4. Similarities Between Sensitivity and Specificity

5. Side by Side Comparison –Sensitivity vs Specificity in Tabular Form

6. Summary

## What is Sensitivity?

Sensitivity, also known as the recall rate, is a measurement that determines the probability of actual positives. In other words, this test feature mainly focuses on identifying the sample members who are actually positive towards the tested property. For instance, let’s take a test which finds how many patients are actually suffering from a certain disease. Then, we can say that we are expecting the probability of positive responses towards the tested property; “ill”. Therefore, such measurements focus on sensitivity. Following is a simple equation that represents the sensitivity.

*Sensitivity = Number of true positives /[ Number of true positives + Number of false negatives]*

Trying to achieve 100% sensitivity in a practical test is quite impossible because it eliminates the portion of incorrectly rejected. Therefore, the effort is to reach very high sensitivity; a high sensitive test is quite reliable. However, one should not think sensitivity means precision. Precision delivers a ratio of positive results to the false positive results, whereas sensitivity is a measure of the ratio of actual positives to the total of positives the test measured, including the indirectly counted ones.

## What is Specificity?

Specificity, also known as true negative rate, is a measurement that determines the probability of actual negatives. The focus of this measurement is to find out the sample members who are actually negative towards the tested property. Taking the same example of testing people for suffering from a certain disease, if the test focuses on measuring the people who do not have the disease, then we can say that the test measures specificity. Therefore, it is clear that what specificity does is confirming how many are negative towards the tested property. Specificity can also be easily put into an equation as below.

*Specificity = Number of true negatives / [Number of true negatives+ Number of false positives]*

Moreover, specificity is very important in medical testing and chemical testing. In medical testing, confirming that a person does not have the disease is more important than detecting whether a person has it. When the positive response is taken into consideration, there is no assurance on the degree of the disease since it simply states that a person is positive. But, knowing a person has no disease is a strong result. It is the same for chemical testing, where finding that certain substances are present is a weak result than finding its absence. Both these statistical properties are important, and it is crucial to decide which should be traded off for which.

## What are the Similarities Between Sensitivity and Specificity?

- Sensitivity and specificity are two statistical measures of a test.
- They are widely used in medicine.
- That is; they measure the probabilities of something tested to be positive or negative.
- Also, both are expressed in percentage values.
- Moreover, achieving 100% sensitivity or 100% specificity is practically difficult.

## What is the Difference Between Sensitivity and Specificity?

Sensitivity measures the probability of actual positives, while specificity measures the probability of actual negatives. Thus, this is the key difference between sensitivity and specificity. In medical tests, sensitivity mainly focuses on finding the people who are suffering from the disease, while specificity mainly focuses on measuring the people who do not have the disease. Therefore, it is also a difference between sensitivity and specificity.

## Summary – Sensitivity vs Specificity

Sensitivity and specificity are two statistical measures we frequently use in medicinal tests. Sensitivity mainly focuses on measuring the probability of actual positives. On the other hand, specificity mainly focuses on measuring the probability of actual negatives. So, this is the key difference between sensitivity and specificity. But in practical applications, 100% sensitivity and 100% specificity are quite impossible.

##### Reference:

1.“Sensitivity and Specificity.” Wikipedia, Wikimedia Foundation, 7 May 2019, Available here.

##### Image Courtesy:

1. “Sensitivity and specificity” By FeanDoe – Modified version from Walber’s Precision and Recall (CC BY-SA 4.0) via Commons Wikimedia

docrings says

I think of it in simple terms for medical tests: “Sensitivity is the likelihood of successfully detecting a true disease”, and “Specificity is the likelihood of successfully detecting a truely normal/disease-free person”.

forensic doc says

In practical usage what would constitute an acceptable level of identification {percentage} of specificity or specificity that would actually allow an evaluator to enhance his/her clinical judgement since 100% is unattanible.